This invention relates to a projection type display apparatus normally called liquid crystal projector in which a liquid crystal light valve is used.
A projection type display apparatus in which a liquid crystal light valve is used is conventionally configured in such a manner as shown in FIG. 11.
Referring to FIG. 11, the projection type display apparatus 90 shown includes an illumination unit (illumination optical system) 10 wherein part of non-polarized white light from a white light source 11 directly passes, but the remaining part of the non-polarized white light is first reflected by a reflector 12 and then passes, through a pair of multi-lens arrays 13 and 14 disposed in an opposing relationship to each other so that they are converted into a light flux which has an intensity distribution uniform within a fixed region on a plane perpendicular to the center axis of the light flux. Further, the light flux is converted from non-polarized light into linearly polarized light by a polarizing conversion element 15 and goes out from the illumination unit 10 through a condenser lens 16.
The light flux going out from the illumination unit 10 is decomposed into red color light 3R, green and blue color light 3GB by a dichroic mirror 21, and the red color light 3R is reflected by a mirror 22 and introduced into a liquid crystal light valve 30R for red through a condenser lens 28R.
Meanwhile, the green and blue color light 3GB is decomposed into green color light 3G and blue color light 3B by a dichroic mirror 23, and the green color light 3G is introduced into a liquid crystal light valve 30G for green through a condenser lens 28G while the blue color light 3B is introduced into a liquid crystal light valve 30B for blue through a relay lens 24, a mirror 25, another relay lens 26, another mirror 27 and a condenser lens 28B.
The liquid crystal light valves 30R, 30G and 30B are each formed from a liquid crystal panel of the transmission type, and images are written into the liquid crystal light valves 30R, 30G and 30B with red, green and blue color signals, respectively.
The decomposed color lights 3R, 3G and 3B of red, green and blue are modulated and converted into image lights 4R, 4G and 4B of red, green and blue by the liquid crystal light valves 30R, 30G and 30B, respectively. The image lights 4R, 4G and 4B of red, green and blue are synthesized by a dichroic prism 40, and the image light 5 after the synthesis is projected in an enlarged scale on a screen 110 by a projection lens 50.
While FIG. 11 shows a display apparatus of the front projection type wherein the optical units from the illumination unit 10 to the projection lens 50 form the projection type display apparatus 90 separately from the screen 110, also optical units of a display apparatus of the back projection type wherein the optical units from the illumination unit to the projection lens and the screen are disposed in a cabinet are formed similarly as in the projection type display apparatus 90 of FIG. 11.
With the conventional projection type display apparatus described above, however, the contrast on the screen cannot be raised higher than a certain fixed value from a view angle of a liquid crystal panel which composes each of the liquid crystal light valves as described below.
FIG. 12 illustrates a manner wherein a light flux from the illumination unit 10 in the projection type display apparatus 90 of FIG. 11 is introduced into a liquid crystal light valve 30 in principle with the decomposition optical system omitted. Here, the liquid crystal light valve 30 is the liquid crystal light valve 30R, 30G or 30B for red, green or blue.
It is to be noted, however, that, while, in the projection type display apparatus 90 of FIG. 11, the lengths of the optical paths from the illumination unit 10 to the liquid crystal light valves 30R, 30G and 30B are set such that the optical paths for the red color light 3R and the green color light 3G are equal to each other but the optical path for the blue color light 3B is longer than those for the red color light 3R and the green color light 3G so that the length is unequal with regard to the blue color, it is assumed that, in FIG. 12, the optical path lengths from the illumination unit 10 to the liquid crystal light valve 30 are equal for the red, green and blue color lights for the convenience of description.
Usually, the size of the light exit of the illumination unit 10 is relatively greater than the aperture size of the liquid crystal light valve 30. Therefore, light emitted from the illumination unit 10 is introduced into the liquid crystal light valve 30 while being condensed, and as viewed from the liquid crystal light valve 30, the light is introduced from every direction within a certain angular range with respect to the direction of a normal 32 to the liquid crystal light valve 30.
However, since TN (Twisted Nematic) liquid crystal of the twisted light mode wherein the major axis of a liquid crystal molecule is twisted by 90xc2x0 between the top and the bottom of the liquid crystal layer is used for a liquid crystal panel which composes the liquid crystal light valve 30, the light shutter characteristic differs depending upon the view angle.
As regards the view angle, as shown in FIG. 13, the angle xcfx86 of outgoing light 33b from the liquid crystal panel 31 originating from incoming light 33a to the liquid crystal panel 31 with respect to a reference azimuth on an outgoing face 34 of the liquid crystal panel 31 is defined as an azimuth angle, and an angle xcex8 of the outgoing light 33b with respect to the normal 32 to the liquid crystal panel 31 is defined as a polar angle. As viewed on the display screen on the outgoing face 34, xcfx86=0xc2x0 corresponds to the rightward direction; xcfx86=90xc2x0 corresponds to the upward direction; xcfx86=180xc2x0 corresponds to the leftward direction; and xcfx86=270xc2x0 corresponds to the downward direction.
FIG. 14 illustrates a result of measurement of the view angle dependency of the light cutoff rate which was performed while a certain liquid crystal panel was placed in an all-black state. The center of concentric circles corresponds to xcex8=0; the circle denoted by numeral 5 corresponds to xcex8=5xc2x0; the circle denoted by numeral 10 corresponds to xcex8=10xc2x0; and the circle denoted by numeral 15 corresponds to xcex8=15xc2x0.
The region indicated as xe2x80x9c99% xcx9cxe2x80x9d is a view angle range within which light less then 1% is transmitted; the region indicated as xe2x80x9c90% xcx9cxe2x80x9d is a view angle range within which light less than 10% is transmitted; the region indicated as xe2x80x9c80% xcx9cxe2x80x9d and surrounded by a thick line is a view angle range within which light less than 20% is transmitted; and the region indicated as xe2x80x9c70% xcx9cxe2x80x9d and surrounded by a broken line is a view angle range within which light less than 30% is transmitted.
Accordingly, with the liquid crystal panel shown, a sufficient light shutter function is exhibited and a high contrast is exhibited for any light flux 37 which penetrates the liquid crystal panel upwardly from below when the liquid crystal panel is viewed downwardly from above as indicated by an arrow mark 35 of FIG. 12 as viewed from the outgoing face 34 side shown in FIG. 13. However, the light shutter function is inferior and the contrast is low for any light flux 38 which penetrates the liquid crystal panel downwardly from above when the liquid crystal panel is viewed upwardly from below as indicated by an arrow mark 36 of FIG. 12.
Since the contrast on the screen is given by an arithmetic mean of contrasts of light fluxes introduced into the liquid crystal light valve 30, with the conventional projection type display apparatus described above, the contrast on the screen cannot be raised higher than a certain fixed value.
In order to solve the problem just described, it is disclosed to use an optical film having an optical anisotropy in Japanese Patent Laid-Open No. 222213. However, the optical film having an optical anisotropy is disadvantageous in that it is complicated in a method of production and is expensive and that the optical anisotropy changes if it is illuminated with intense light.
It is an object of the present invention to provide a projection type display apparatus which can maintain a high contrast for a long period of time and can be produced simply and at a low cost.
In order to attain the object described above, according to the present invention, there is provided a projection type display apparatus, including an illumination optical system for emitting light from a white light source as a light flux whose intensity distribution is uniformed, a decomposition optical system for decomposing the emitted light flux into color lights of red, green and blue, a plurality of liquid crystal light valves for the colors into which the decomposed color lights of the colors are introduced individually, a synthesis optical system for synthesizing the image lights of the colors emitted from the liquid crystal light valves for the colors, and a projection optical system for projecting the synthesized image lights to a screen, wherein a first optical path from the illumination optical system to one of the liquid crystal light valves for a predetermined one color through the decomposition optical system is formed longer than second optical paths to the liquid crystal light valves for the other two colors, and a filter is disposed at one of positions opposing to each other across the center of the light flux between the white light source and the decomposition optical system for attenuating or cutting off light of a wavelength region different from that of light which passes the other of the positions.
The projection type display apparatus is advantageous in that a high contrast can be maintained for a long period of time and it can be produced simply at a low cost.
The above and other objects, features and advantages of the present invention will become apparent from the following description and the appended claims, taken in conjunction with the accompanying drawings in which like parts or elements denoted by like reference symbols.